Recently, there has been a growing interest in directly modulated lasers for use as transmitters in optical communication systems because they allow a compact design, have large response to modulation, and are integrable. In addition, they are typically inexpensive compared to externally-modulated transmitters, which require an intensity modulator, such as LiNbO3 modulators, following the laser.
Various optical sources, such as semiconductor lasers, exhibit optical frequency chirp when directly modulated by a signal, due to the fact that intensity modulation in semiconductor lasers is invariably accompanied by phase modulation because of the carrier-induced change in the refractive index.
Fiber based transmission systems that use Directly Modulated Laser (DML) sources having optical frequency chirp suffer signal degradation due to optical dispersion in the transmission fiber. In a digital system, dispersion causes the digital pulses to spread in time. As a result, the pulses can overlap and interfere with each other, thereby limiting data transmission speed. Since pulse duration broadens linearly with traveled distance, system performance degrades because of Inter Symbol Interference (ISI). For instance, DML sources at 1550 nm wavelength and 10 Gb/s data rate typically generate an optical signal that propagates no more than 10 kilometers in standard fiber links, such as along standard single-mode fibers (SMFs).
In order to minimize the transient response of the DML occurring at bit transitions, conventional direct intensity modulation of a semiconductor laser is achieved by operating the laser with a bias current well above the lasing threshold and setting a modulation depth small enough to avoid switching off of the laser upon modulation, i.e., by operating in the so-called small or weak signal regime. In this condition, the “off” or “0” state has an optical power (P0) that is a fraction of the power (P1) at the “on” or “1” state. In addition the small modulation mitigates the inclusion in the signal of the transient and thermal chirp contributions arising from the sudden changes in injection current. However, this choice affects the value of Extinction Ratio (ER), defined as ER=P1/P0 and often also specified as a dB value, 10 log(ER), which is typically not larger than about 2 dB.
In an optical communication system, the receiver sensitivity, expressed in terms of received optical power, increases as the ER decreases. This is because any deviation from the ideal optical signal at the receiver, i.e. a bit stream made of bits ‘1’ bits ‘0’, leads to a change of the ‘0’ and ‘1’ bit levels, reducing the degree of discrimination between the two symbols and leading to ISI. To keep the Bit Error Rate (BER) at a predetermined value, the minimum average optical power required at the receiver increases because of such non-ideal conditions. This increase in the average received power is generally referred to as power penalty.
Larger optical link lengths can be achieved by employing frequency modulation of the laser with subsequent optical conversion into intensity modulation.
A frequency discriminator may be chosen to partially compensate for the dispersion in the transmission fiber while converting a Frequency Modulated (FM) signal from a laser into a substantially intensity-modulated signal, generally referred to as Amplitude Modulated (AM) signal.
What is needed, therefore is an optical system and method that overcome at least the shortcomings described above.